CARET is an ongoing double-blind lung cancer chemoprevention trial of the efficacy and safety of the combination of 30 mg beta-carotene and 25,000 IU retinyl palmitate daily in two high-risk populations: (a) male and female current and former smokers recruited primarily from insurance-based sources, and (b) males with extensive occupational asbestos exposure recruited primarily from employment-based sources. We have documented successful recruitment, excellent compliance, and minimal side effects in over 18,000 participants randomized thus far at six study centers. Through 30 June, 1994, CARET has randomized 4,060 asbestos-exposed workers, exceeding accrual goals at all five CARET asbestos centers, and 14,057 heavy smokers (44% women). Two of the three smoker centers exceeded their goals, the third will conclude randomizations in fall 1994. During the final five-year period, CARET will focus on retention, adherence to protocol, ascertainment of endpoints, monitoring of key design parameters, closeout of the trial, and analysis and publication. Projections indicate that with 14,240 smokers and 4,060 asbestos-exposed participants and 114,400 person- years of follow-up through February 1998, CARET will be capable of detecting a 23% reduction in lung cancer incidence in the two high- risk populations combined, and 27%, 49%, 31%, and 35% reductions in the smokers, female smokers, male smokers, and asbestos-exposed subgroups, respectively. The Coordinating Center continues to be responsible for all statistical work, close liaison with study centers, data management and data operations, progress reports, purchase and quality-controlled distribution of vitamins, serum and DNA bank, central laboratory, review and adjudication of endpoints data, publications and presentations, and general administration. CARET is highly complementary to the ATBC (alpha-tocopherol, beta-carotene) study of 29,000 male smokers in Finland and the Harvard Physicians Health Study (beta-carotene alone) of 22,000 men in the NCI portfolio of major cancer chemoprevention trials.